The use of thermally conductive cores for transfer of heat generated by a heat source to a heat sink is well known in the art. Such cores have historically been comprised of relatively high thermal conductivity metals such as copper or aluminum. Unfortunately, these high thermal conductivity metals also have high thermal coefficients of expansion that adversely affect their permissible applications. For example, if a copper or aluminum thermal core is mounted to a printed circuit board to transfer the heat generated by attached circuit components to a heat sink for dissipation, the expansion and contraction of the core due to thermal changes in the transferred heat tends to warp the circuit board resulting in fatigue and failure of solder joints and/or cracking of the board substrate. In other applications, the use of copper or aluminum as a thermal transfer means is not permitted because cores manufactured from these metals tend to exceed maximum weight specifications.
One solution to the problem of thermal expansion and contraction has been to use low coefficient of thermal expansion metals such as invar, kovar and/or molybdenum in conjunction with copper or aluminum. By layering or laminating the invar, kovar or molybdenum with the copper or aluminum, the low coefficient metals tend to control the thermal expansion and contraction difficulties experienced with the high coefficient metals, producing a more thermally stable core. Unfortunately, the use of these low coefficient of thermal expansion metals adversely affects the overall thermal conductivity performance of the laminated or layered core in comparison to a conventional copper or aluminum core. Furthermore, these layered or laminated cores suffer from the same weight concerns experienced with copper or aluminum cores and are therefore not favored in airborne and aerospace applications.
The most recent evolutionary step in thermal core construction techniques has been the use of fiber reinforced composites that possess the benefit of being highly thermally conductive (two to three times greater than copper) while simultaneously being relatively lightweight in comparison to conventional cores. Several drawbacks with composite cores have been noted. The fibers used in composite cores are highly thermally conductive in the direction along their length but exhibit poor thermal conduction characteristics orthogonal to the direction of each fiber. Furthermore, the binder used to secure the fibers together in the core possesses relatively low thermal conductivity characteristics. Thus, composite cores inefficiently conduct generated heat transverse to the fiber direction. For most efficient operation of the composite fiber core, the fiber ends must be placed adjacent to the heat source or sink.
Two methods have been employed to bring the fiber ends adjacent the source or sink. First, as disclosed in U.S. Pat. Nos. 4,867,235 and 4,849,858, issued to Grapes, et al., the composite core is bent such that the fiber ends are inclined with respect to the core surface to facilitate placement of the fiber ends adjacent the source or sink. This core construction method is not preferred because bending of the composite core is expensive and may result in breakage of the fibers and/or damage to the structural integrity of the composite core. A second construction method, disclosed in U.S. Pat. No. 5,111,359, issued to Montesano and assigned to the assignee of this application for patent, requires the machining of the core to form an opening (exposing the fibers therein) having a precise shape for receiving and adhesively mounting a thermally conductive wedge. The drawbacks experienced with this fabrication method are increased costs due to the precise tolerances required to mate the wedge to the opening and inefficiencies in heat transfer between the fibers and the inserted wedge resulting from the adhesive mounting of the wedge. Accordingly, there is a need for an improved thermal transfer substrate design that is less expensive to manufacture and more efficient to operate than known prior art substrates.